In the spirit of a lounge conversation, I would like to point out that other religions do this too: I know of at least Hindu and Buddhist examples. In the Buddhist case, the reason is that one is giving up one's lay existence with all the attendant duties as a member of a family and recognition in society and becoming subservient to the monastic entity and forming a part of that. The reasons are different but similar in Hinduism: the teacher that takes the new name is no longer the person with the ego that was known by the old name, he or she is a new person endowed with a quality of rightness in their thought and teachings which transcends the individuality of the person with their birth name.

Law in India recognizes the change in name, but since the religious purpose of the change is to renounce the individuality of the former existence, the law applies to the extent that secular society does not grant the religious freedom of complete renunciation of the past.